1. Field of the Invention
The present invention relates to an apparatus and method for analyzing particles in a continuous operation even when particles are not present, and more particularly, concerns an apparatus and method for regulating sheath fluid flow in a hydrodynamically focused fluid flow system.
2. Description of the Prior Art
There are a number of cell or particle analyzing devices employing flow cytometry techniques which rely on hydrodynamically focused fluid flow through an aperture for determining specific characteristics of the flowing cells or particles. Flow analysis of particles has been employed in the determination of a variety of characteristics of individual particles. This analysis is most useful in analyzing or determining characteristics of cells for the collection of information which would be useful in areas of research, hematology, immunology and the like. The researcher, for example, may be interested in determining specific characteristics of individual cells so that such cells may be classified, identified, quantified, and perhaps sorted for further investigations or analysis.
Three devices which rely on hydrodynamically focused fluid flow systems are sold by Becton, Dickinson and Company. One device, known as the ULTRA-FLO 100.TM. Whole Blood Platelet Counter, rapidly and reliably counts whole blood platelets in the hematology laboratory. In the ULTRA-FLO 100 system, a trajectory of a diluted sample containing platelets passes straight through the center of the counting chamber orifice since the sample fluid is focused by a sheath of pressurized fluid. Another device sold by Becton, Dickinson and Company, relying on a hydrodynamically focused fluid flow system, is known as the FACS.TM. analyzer. The FACS analyzer rapidly analyzes cells on the basis of fluorescence and electronic volume properties. Analysis is accomplished by introducing cells in suspension to the center of a focused liquid stream and causing them to pass, one at a time, through the filtered and focused light from a high-power mercury-arc lamp. Each cell is individually characterized by its electronic impedance volume and by the intensity and color of fluorescence emitted while it is illuminated. Another device known as the FACS.TM. sorter operates on principles similar to the FACS.TM. analyzer, but further sorts the cells based on specifically detected characteristics. In all of the aforementioned systems, a sheath fluid is utilized to focus the particles or cells as they pass through the aperture associated with the analyzing or counting capabilities. U.S. Pat. Nos. 4,240,029, 4,165,484 and 4,110,604 describe particle analysis systems in which particles flowing in a stream are enveloped in a sheath fluid which focuses and confines the sample fluid (with particles) to the center of the flowing stream.
When a sheath fluid is used in a hydrodynamically focused fluid flow system, the sheath fluid is normally stored in a reservoir from which it is withdrawn during the operation of the device to ensheathe the sample fluid having the particles to be analyzed contained therein. Oftentimes in these types of analysis systems or equipment, the apparatus is turned on either just prior to or at the time the first sample of fluid is to be tested. When a sample has been tested in the analyzer, counter or the like, the sample is normally removed while the equipment remains operating. If the equipment is operating continuously, the sheath fluid normally flows at the same rate irrespective of whether a sample fluid is being tested or not. As a result, during changes between samples or during any delays, sheath fluid might normally drain from the equipment when the sample is not in position. This causes the supply of sheath fluid in the reservoir to become depleted more quickly and without benefit if it drains from the equipment when a sample is not being tested. Of course, the sheath fluid may be collected as it drains from the equipment for subsequent use. However, this could require the shut down of the equipment to restore the collected sheath fluid back to the reservoir for such subsequent use. Filtering the fluid can also be accomplished so that the fluid can be re-used in the equipment.
U.S. Pat. No. 4,070,617 discloses an apparatus for controlling the particle flow when measuring the properties of particles suspended in liquid. This patented invention includes a container for storing the particle-free electrolyte and a drip chamber into which fluid from the container drips. The drip chamber is adjustable thereby allowing an adjustment in the pressure difference between the particle suspension liquid and the particle-free electrolyte carrier. By adjusting this pressure differential, flowrate of the particle suspension into the electrolyte carrier can be controlled. However, the invention of U.S. Pat. No. 4,070,617 does not satisfy the needs of a system such as described above. U.S. Pat. No. Re. 28,558 discloses an apparatus for studying physical properties of particles passing through a screening aperture and carried in suspension, wherein a sheath fluid is employed and is re-used in the closed, recirculating system.
It is desirable to have a minimum change in the fluid level in the sheath reservoir of a hydrodynamically focused system particularly since the variations in the sheath fluid level affect the differential pressure between the sheath and sample fluid, which in turn affects the sample flow rate. Further, it would also be desirable to provide a system where the sheath flow rate is reduced or minimized during periods when no sample is present for analysis, but while the equipment is still operating. It is toward the satisfaction of the aforementioned desired achievements that the present invention is directed.